The invention relates to a method of selecting television display tubes in which the vacuum is too low during manufacture.
In the manufacture of television display tubes, colour display tubes in particular, it is important that, during the manufacturing process, all the display tubes having too low a vacuum are timely removed, so that they are not changed such that during further manufacturing steps, for example during gettering, that repair/recycling of these tubes is rendered more difficult of even impossible.
To detect whether a tube has a vacuum which is too low it has been customary to apply a pulsed high voltage between two electrodes which are fed-out from the tube base, for example the grids (g2) and (g3), and to measure the resulting ionization current.
This method is comparatively unreliable because the insulating currents within the tube base are included in the measured currents.